In a field of optical pickup apparatus, conversion into a shorter wavelength of a laser light source used as a light source for reproducing of information recorded on an optical disk, and for recording information on an optical disk has been advanced in recent years. A laser light source with wavelength 405 nm such as a violet semiconductor laser and a violet SHG laser wherein a wavelength of an infrared semiconductor laser is converted by the use of second harmonic generation, for example, have been put to practical use.
When these violet laser light sources are used, information of 15-20 GB can be recorded in an optical disk having a diameter of 12 cm in the case of using an objective lens having a numerical aperture (NA) that is the same as that of DVD (digital versatile disc), and when NA of the objective lens is enhanced to 0.85, information of 23-25 GB can be recorded for the optical disk having a diameter of 12 cm. Hereafter, in the present specification, an optical disk using a violet laser light source and a magneto-optical disk are generically called “a high density optical disc”.
Incidentally, in recording and/or reproducing the high density optical disc using an objective lens having NA 0.85, comatic aberration caused by an inclination (skew) of the optical disk is increased, and therefore, an amount of comatic aberration caused by a skew is reduced by designing a protective layer to be thinner than in the case of DVD (to be 0.1 mm when the protective layer for DVD is 0.6 mm).
However, only a capability to conduct recording and reproducing of information properly for the high density optical disc is not a sufficient value as a product of an optical disk player and a recorder. In the present time, when it is taken into consideration that DVD and CD (compact disc) on which various types of information are recorded actually are on the market, only a capability to conduct recording and reproducing of information for the high density optical disc is not sufficient, and a capability, for example, to conduct recording and reproducing of information equally and properly also for DVD and CD already owned by a user leads to enhancement of a commercial value as an optical disk player for a high density disc and a recorder. From such background, the optical pickup apparatus equipped on the optical disk player for a high density disc and a recorder is required to have capability to conduct recording and reproducing of information properly while maintaining compatibility for any of a high density optical disc, DVD and further CD.
As a method to conduct recording and reproducing of information properly while maintaining compatibility for any of a high density optical disc, DVD and further CD, there is considered a method to selectively switch an optical system for a high density optical disc and an optical system for DVD and CD, depending on recording density of the optical disk to be subjected to recording and reproducing in terms of information. However, this method requires a plurality of optical systems, which is disadvantageous for downsizing and it causes cost increase.
Therefore, for achieving low cost by simplifying the structure of an optical pickup apparatus, it is preferable to provide a common optical system for a high density optical disc and that for DVD and CD even in the optical pickup apparatus having compatibility, and thereby to reduce, to the Utmost, the number of optical parts which constitute the optical pickup apparatus. Providing common objective optical systems which are arranged to face the optical disk is most advantageous for simplification of the structure of the optical pickup apparatus and for low cost thereof. Incidentally, for obtaining common objective optical systems for plural types of optical disks wherein wavelengths for recording and reproducing are different, it is necessary to form a phase structure having wavelength-dependency for spherical aberration on the objective optical system.
In Patent Document 1, there are described an objective optical system which is of the two-group structure having a diffractive structure as a phase structure and can be used commonly for the high density optical disc and for conventional DVD and CD and an optical pickup apparatus that is equipped with the objective optical system.
The objective optical system which is of the two-group structure described in Patent Document 1 has the structure wherein a working distance for the optical disk having thick protective layer such as DVD and CD is secured by making a light converging element on the optical disk side to shoulder the most of paraxial power, and vignetting of a ray of light by a portion of steps of the diffractive structure is prevented and transmittance is improved by forming, on an aberration correcting element on the light source side, a diffractive structure as a phase structure.
(Patent Document 1) European Patent Application Publication No. 1304689
For manufacturing efficiently an efficient two-group-structured compatible objective system, it is necessary to obtain lenses each being kept within an aberration range at a certain extent and to put these lens groups together. When aberration of each lens group is great, problems stated below are caused in an assembling process.
For example, when large comatic aberrations remain in each lens group, it is necessary to eliminate these comatic aberrations through adjustment in assembly, to make the lens group to be used as the compatible objective optical system. However, even in the case where the comatic aberration is corrected for the wavelength on one side through adjustment in assembly, there still is a high possibility that the comatic aberration still remains for the wavelength on the other side, and the lens group cannot be used as the compatible objective optical system.
Further, the aforesaid confirmation of comatic aberration for all working wavelengths for assembly of each lens group lowers manufacturing efficiency conspicuously, which leads to an increase of manufacturing cost, and causes a fear to preclude mass production.
The objective optical system described in the Patent Document 1 does not disclose any solution for the problems mentioned above. Therefore, a two-group-structured compatible objective optical system having sufficient performance for plural working wavelengths cannot be manufactured efficiently and at low cost.